The present invention relates to the field of global positioning, and more particularly to an apparatus and method of deriving a GPS time estimate from information supplied by a wireless communications system for use by a positioning system receiver associated with a wireless communications mobile terminal, such as a cellular phone, so as to improve at least time-to-first-fix and signal sensitivity.
Satellite positioning receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites such as the U.S. Global Positioning System (GPS) or NAVSTAR satellites. For example, the GPS Orbital Constellation consists of 28 satellites which orbit the earth in 12 hour orbits. The satellites are arranged in six orbital planes each containing four or more satellites. The orbital planes are spaced 60xc2x0 apart from each other and are inclined approximately 55xc2x0 with respect to the equatorial plane. This constellation provides a user with approximately four to twelve satellites visible from any point on earth. These satellites transmit, as part of their message, both satellite positioning data, so-called xe2x80x9cephemerisxe2x80x9d data, as well as clock timing data. In addition, the satellites transmit time-of-week (TOW) information associated with the satellite signal, which allows the receiver to unambiguously determine time. The process of searching for and acquiring GPS signals, reading the ephemeris and other data for a multiplicity of satellites, and then computing the location of the receiver (and accurate time-of day) from this data is time consuming, often requiring several minutes. In many applications, this lengthy processing time introduces unacceptable delays, and furthermore, greatly limits battery life in portable applications.
In addition, in many situations there may be blockage of the satellite signals. In these cases, the received signal level from the GPS satellites can be too low to demodulate and derive the satellite data signals without error. Such situations may arise in personal tracking and other highly mobile applications. In these situations, it may not be possible for a GPS receiver to acquire and track the GPS signals without first having a very good estimate of the actual GPS time.
Despite the difficulties associated with GPS positioning, it is becoming increasingly common to incorporate a GPS receiver into a wireless communications mobile terminal. This combination may be advantageously leveraged to provide assistance to the GPS receiver in determining time with the assistance of time information supplied by the wireless communications system. Such an arrangement is discussed in U.S. Pat. No. 5,945,944 to Krasner which discusses the idea in the context of a variety of wireless communications systems, including Time Division Multiple Access (TDMA) networks operating according to the standard known as Global System for Mobile Communications (GSM). The Krasner approach, however, may not function adequately when the mobile terminal is engaged in a call on a traffic channel, particularly when the traffic channel does not contain a firm reference to GPS time and when the traffic channel is not necessarily synchronized to the last used control channel.
Thus, there remains a need for an apparatus and method of providing time information assistance to a GPS receiver via a wireless communications system mobile terminal during active call sessions in a system where traffic channels are not necessarily synchronized to control channels.
The present invention meets this and other needs by providing a method and apparatus for acquiring timing signals for use in a positioning receiver based on timing assistance provided by a wireless communications system, such as cellular telephone system. In particular, this invention provides an accurate GPS time estimate based on a relationship between GPS time-of-week (TOW) and a particular point in the long-term cycle of transmissions from the mobile wireless communications network with which the GPS-equipped mobile terminal communicates. This relationship is provided to the mobile terminal as aiding information (xe2x80x9cassistancexe2x80x9d) from the radio network, perhaps accompanied by other assistance representative of satellite locations, etc.
In an illustrative example, the mobile terminal operates in a wireless communications system having control channels and at least one traffic channel unsynchronized to the control channels. The mobile terminal operates in an idle (non-call) state on a first control channel in a first cell having a first time base associated therewith. While receiving the control channel, the mobile terminal establishes a local clock reference, internal to the mobile terminal, capable of deriving the first time base. The mobile terminal then operates on a first traffic channel that is unsynchronized to the first control channel and notes the difference in time bases between the control channel and the traffic channel. If the mobile terminal then moves to another traffic channel, the mobile terminal preferably also notes the difference in time bases between the traffic channels, so as to relate the time base of the present traffic channel to the time base of the control channel. Either while receiving the control channel in idle state or while operating on the traffic channel, the mobile terminal receives TDMA-to-GPS reference data, which indicates the relationship between system time for the wireless communications system as expressed on particular control channel (xe2x80x9cTDMA timexe2x80x9d) and the system time for the GPS system (xe2x80x9cGPS timexe2x80x9d). For instance, the TDMA-to-GPS reference data may say that at particular point A in the TDMA time base, the corresponding GPS time was B. Thereafter, an accurate estimate of the GPS time is calculated based on the local clock reference and the TDMA-to-GPS relationship data. In some preferred aspects, the local clock reference is updated based on the first traffic channel so as to combat drift.